The present invention relates to a semiconductor light-emitting element.
In recent years, semiconductor light-emitting elements are widely used in an outdoor display, automobile indicator, and the like. The semiconductor light-emitting element is a device using emission recombination of electrons and holes injected in a p-n junction region. Emission ranging from infrared radiation to ultraviolet radiation can be realized by changing the semiconductor material of a light-emitting layer.
FIG. 30 shows the structure of a conventional semiconductor light-emitting element. An n-type GaAs buffer layer 3202, an n-type DBR (Distributed Bragg Reflector) reflective layer 3203 made of InGaAlP and GaAs to reflect light using the Bragg reflection effect, an n-type InGaAlP cladding layer 3204, an active layer 3205, a p-type InGaAlP cladding layer 3206, a p-type AlGaAs window layer 3207, and a p-type GaAs contact layer 3208 are sequentially formed on the upper surface of an n-type GaAs substrate 3201.
An n-type electrode 3209 is formed on the lower surface of the n-type GaAs substrate 3201, and a p-type electrode 3210 is formed on the p-type GaAs contact layer 3208. Power is supplied to the light-emitting element to emit light from the active layer 3205. Light emitted downward in FIG. 30 by the active layer 3205 is reflected by the reflective layer 3203, and radiated to above the element via the window layer 3207 together with the light emitted upward.
The conventional semiconductor light-emitting element suffers the following problem.
Part of light that is emitted downward by the active layer 3205 and travels straight toward the reflective layer 3203 is reflected by the reflective layer 3203 without being absorbed by the substrate 3201, and can be effectively extracted outside.
However, the reflective layer 3203 exhibits a very low reflectivity with respect to light traveling diagonally toward the reflective layer 3203, so not all the light from the active layer 3205 can be extracted outside.
The semiconductor light-emitting element absorbs light by a substrate which provides a critical angle defined by the difference in refractive index between the semiconductor crystal and the atmosphere or enables crystal growth. For this reason, light which can be extracted outside is only several % of internally emitted light.
FIG. 26 shows the structure of another semiconductor light-emitting element relating to the present invention.
A multilayered reflective film 1001, p-type contact layer 1002, p-type cladding layer 1003, active layer 1004 functioning as a light-emitting layer, n-type cladding layer 1005, and n-type contact layer 1006 are formed on a p-type semiconductor substrate 1000. An n-type electrode 1007 is formed on the contact layer 1002, whereas a p-type electrode 1008 is formed on the contact layer 1006.
Part of light emitted by the active layer 1004 that travels toward the n-type cladding layer 1005 is extracted outside via the cladding layer 1005.
Light that travels toward the p-type cladding layer 1003 is reflected by the multilayered reflective film 1001, and extracted outside via the n-type cladding layer 1005.
In this structure, light emitted toward the substrate 1000 can be reflected by the reflective film 1001, and extracted outside.
However, the reflectivity of light which is not vertically incident on the reflective film 1001 is low, the electrodes 1007 and 1008 which shield light exist on the light extraction surface, and the active layer 1004 is formed on the reflective film 1001. This results in low crystallinity and short service life.
FIG. 27 shows still another semiconductor light-emitting element relating to the present invention. An n-type InGaP buffer layer 1102, n-type InAlP cladding layer 1103, InGaAlP active layer 1104 functioning as a light-emitting layer, p-type InAlP cladding layer 1105, and p-type GaAs contact layer 1106 are formed on the upper surface of an n-type GaP substrate 1101. A p-type electrode 1107 is formed on the p-type GaAs contact layer 1106, while an n-type electrode 1100 is formed on the lower surface of the substrate 1101.
Light emitted by the InGaAlP active layer 1104 is reflected by the n- and p-type electrodes 1100 and 1107, and extracted outside from a region of the contact layer 1106 which is not shielded by the p-type electrode 1107.
In this structure, however, light concentrated immediately below the electrode 1107 is shielded by the electrode 1107, and cannot be extracted outside.
In the element shown in FIG. 27, only several % of light emitted by the active layer 1104 can be extracted outside owing to the difference in refractive index between the crystal and the air.
As the semiconductor light-emitting element, a compound semiconductor light-emitting element using a GaAs-based semiconductor material is adopted to emit light ranging from red to green, and a gallium nitride-based compound semiconductor light-emitting element using Al(x)Ga(y)In(1xe2x88x92xxe2x88x92y)N (0xe2x89xa6x, yxe2x89xa61, x+yxe2x89xa61) is adopted to emit light from the ultraviolet range to the blue/green range.
However, the refractive indices of these light-emitting elements are high (GaN=2.67, GaAs=3.62), their critical angles are small (GaN=21.9xc2x0, GaAs=16.0xc2x0), and thus their light extraction efficiencies are low.
The GaAs system exhibits large light absorption on the substrate. Emitted light is absorbed by the substrate to decrease the light extraction efficiency.
FIG. 29 shows still another semiconductor light-emitting element relating to the present invention.
An n-type GaAs buffer layer 1301, n-type InGaAlP cladding layer 1302, InGaAlP active layer 1303, p-type InGaAlP cladding layer 1304, and p-type AlGaAs current diffusion layer 1305 are sequentially grown on the upper surface of an n-type GaAs substrate 1300. A p-side electrode pad 1307 is formed on the p-type AlGaAs current diffusion layer 1305, whereas an n-side electrode 1306 is formed on the lower surface of the n-type GaAs substrate 1300.
In this structure, a current flowing from the p-side electrode 1307 is widened by the p-type. AlGaAs current diffusion layer 1305, and injected from the p-type InGaAlP cladding layer 1304 to the InGaAlP active layer 1303. The light is extracted outside the element via the p-type AlGaAs current diffusion layer 1305.
In the GaAs-based compound semiconductor light-emitting element having this structure, part of light emitted by the active layer 1303 that travels toward the substrate 1300 is absorbed by the substrate 1300, and cannot be extracted outside the element. More specifically, 50% of the emitted light cannot be extracted, which is fatal to high luminance.
As described above, the elements relating to the present invention suffer low light extraction efficiency.
The present invention has been made in consideration of the above situation, and has as its object to provide a semiconductor light-emitting element capable of efficiently extracting light emitted by a light-emitting layer outside the element.
According to the present invention, there is provided a semiconductor light-emitting element comprising a substrate, a reflective layer which is formed on the substrate, contains a metal, and reflects light, a light-emitting layer formed on the reflective layer to emit light, and a transparent electrode formed on the light-emitting layer to transmit light.
The light-emitting layer desirably has a double-heterostructure in which an active layer is sandwiched between first and second cladding layers.
The semiconductor light-emitting element can further comprise an electrode of one conductivity type between a surface of the substrate and the reflective layer, a contact layer of the one conductivity type between the reflective layer and the light-emitting layer, and a contact layer of an opposite conductivity type between the light-emitting layer and the transparent electrode.
The semiconductor light-emitting element can further comprise an electrode of one conductivity type between a surface of the substrate and the reflective layer, a contact layer of the one conductivity type between the reflective layer and the light-emitting layer, and a contact layer of an opposite conductivity type between the light-emitting layer and the transparent electrode.
The semiconductor light-emitting element can further comprise a strain relaxing layer which is sandwiched between the contact layer of one conductivity type and the first cladding layer, and has a middle band gap between band gaps of the contact layer of the one conductivity type and the first cladding layer.
The contact layer of the one conductivity type and the contact layer of the opposite conductivity type may contain InGaP or InGaAlP.
The semiconductor light-emitting element can further comprise an intermediate layer between the electrode of the one conductivity type and the reflective layer.
The reflective layer may have a two-layered structure made up of a transparent conductive film and a metal.
The transparent electrode may be formed using an ITO film.
If the substrate contains a metal, preferable heat dissipation can be attained.
Compositions of first and second cladding layers are desirably adjusted to set band gaps of the first and second cladding layers to be larger than a band gap of an active layer.
An active layer may have a single or multiple quantum well structure including a well layer and a barrier layer.
According to the present invention, the reflective layer made of a metal can obtain a high reflectivity regardless of the angle of incident light on the reflective layer. Light emitted inside the element can be efficiently extracted outside.
A semiconductor light-emitting element according to the present invention comprises a transparent semiconductor substrate, a buffer layer formed on the semiconductor substrate and lattice-matched with the semiconductor substrate, a light-emitting layer formed on the buffer layer, a first electrode formed on the buffer layer, and a second light-reflecting electrode formed on the light-emitting layer.
According to the present invention, light is extracted from the transparent substrate to increase the light extraction efficiency and luminance. The buffer layer lattice-matched with the substrate can prolong the service life with high crystallinity.
A semiconductor light-emitting element according to the present invention comprises a semiconductor substrate, a light-emitting layer formed on the semiconductor substrate, and first and second electrodes formed on the same plane, wherein the semiconductor substrate has a light extraction window so as to pass light emitted by the light-emitting layer.
The first and second electrodes are formed on the same plane. One of these electrodes can be directly formed on a heat sink to increase the luminance without saturating a light output up to a large current.
A semiconductor light-emitting element manufacturing method according to the present invention comprises the steps of forming a buffer layer on a transparent semiconductor substrate so as to be lattice-matched with the semiconductor substrate, sequentially forming a first contact layer, a first cladding layer, a light-emitting layer, a second cladding layer, and a second contact layer on the buffer layer, partially removing the first cladding layer, the light-emitting layer, the second cladding layer, and the second contact layer to expose a surface of the first contact layer, forming a first electrode on the exposed surface of the first contact layer, and forming a second light-reflecting electrode on a surface of the second contact layer.
A semiconductor light-emitting element manufacturing method according to the present invention comprises the steps of sequentially forming a buffer layer, a first contact layer, a first cladding layer, a light-emitting layer, a second cladding layer, and a second contact layer on a semiconductor substrate, partially removing the first cladding layer, the light-emitting layer, the second cladding layer, and the second contact layer to expose a surface of the first contact layer, forming a first electrode on the exposed surface of the first contact layer, forming a second light-reflecting electrode on a surface of the second contact layer, and forming a light extraction window at a portion of the semiconductor substrate at which the light extraction window faces the second electrode.
A semiconductor light-emitting element according to the present invention comprises a transparent semiconductor substrate, a double-heterostructure which is formed on the semiconductor substrate and contains a light-emitting layer and first and second cladding layers that sandwich two surfaces of the light-emitting layer, and a contact layer which is formed on the double-heterostructure and has a recessed surface.
Since the recessed region is set on the contact layer formed on the transparent substrate, light from the light-emitting layer can be reflected to the side surface or the like, and effectively extracted outside the element. Thus, the light extraction efficiency increases.
A semiconductor light-emitting element manufacturing method according to the present invention comprises the steps of sequentially forming a buffer layer, a first cladding layer, a light-emitting layer, a second cladding layer, and a contact layer on a transparent semiconductor substrate, recessing a surface of the contact layer, forming a first light-reflecting electrode on the surface of the contact layer, and forming a second electrode on a surface of the semiconductor substrate so as to remove a portion at which the second electrode faces the first electrode.
Alternatively, a semiconductor light-emitting element manufacturing method according to the present invention comprises the steps of forming a buffer layer on a transparent semiconductor substrate so as to be lattice-matched with the semiconductor substrate, sequentially forming a first cladding layer, a light-emitting layer, a second cladding layer, and a contact layer on the buffer layer, recessing a surface of the contact layer, forming a first light-reflecting electrode on the surface of the contact layer, and forming a second electrode on a surface of the semiconductor substrate.
A semiconductor light-emitting element according to the present invention comprises at least a light-emitting layer formed on a semiconductor substrate, wherein a shape of the semiconductor light-emitting element is a polygonal prism having at least five corners or a circular cylinder.
Since the element shape is a polygonal prism or circular cylinder, light reflected by the end face is reduced, compared to a quadrangular prism. Light inside the element can be effectively extracted outside from the end face to increase the light extraction efficiency.
A semiconductor light-emitting element according to the present invention having a light-emitting layer for emitting light in a direction of plane comprises a photonics crystal layer on at least one surface of the light-emitting layer.
The photonics crystal layer may be formed on the light-emitting layer on a side of a compound semiconductor light-emitting element opposite to a light extraction surface.
Alternatively, the photonics crystal layer may be formed on the light-emitting layer on a light extraction surface side of the semiconductor light-emitting element, and a through dislocation may exist on the light extraction surface in a substantially vertical direction to pass light emitted by the light-emitting layer.
A semiconductor light-emitting element according to the present invention comprises a semiconductor substrate, a contact layer formed on the semiconductor substrate, a first cladding layer formed on the contact layer, a light-emitting layer formed on the first cladding layer, and a second cladding layer formed on the light-emitting layer, wherein an interface of the contact layer in contact with the first cladding layer is corrugated to have a gradient index, and light emitted by the light-emitting layer is reflected by the interface.
A semiconductor light-emitting element according to the present invention comprises a semiconductor substrate, and a light-emitting layer formed on the semiconductor substrate, wherein the semiconductor substrate has a rounded edge.
Alternatively, a semiconductor light-emitting element according to the present invention comprises a photonics crystal layer, and at least one light-emitting element formed on each of two surfaces of the photonics crystal layer, wherein the light-emitting elements emit light with different emission wavelengths.
A semiconductor light-emitting element according to the present invention comprises a transparent semiconductor substrate, a Bragg reflective layer formed on the semiconductor substrate, an active layer formed on the Bragg reflective layer, and a photonics crystal layer formed on the active layer.
A semiconductor light-emitting element manufacturing method according to the present invention comprises the steps of sequentially forming a buffer layer, a first cladding layer, a light-emitting layer, and a second cladding layer on a first semiconductor substrate, forming a photonics crystal layer on the second semiconductor substrate, fusing the second cladding layer and the photonics crystal layer, and removing the first semiconductor substrate and the buffer layer.
Alternatively, a semiconductor light-emitting element manufacturing method according to the present invention comprises the steps of sequentially forming a buffer layer, a contact layer, a first cladding layer, a light-emitting layer, and a second cladding layer on a first transparent semiconductor substrate, forming a photonics crystal layer on a second semiconductor substrate, fusing the first semiconductor substrate and the photonics crystal layer, and removing the second semiconductor substrate, wherein the photonics crystal layer contains a through dislocation on a light extraction surface in a substantially vertical direction to pass light emitted by the light-emitting layer.
Alternatively, a semiconductor light-emitting element manufacturing method according to the present invention comprises the steps of forming a contact layer on a semiconductor substrate, corrugating a surface of the contact layer, and sequentially forming a first cladding layer, a light-emitting layer, and a second cladding layer on the contact layer, wherein a gradient index is given by the corrugated interface of the contact layer in contact with the first cladding layer, and light emitted by the light-emitting layer is reflected by the interface.
A semiconductor light-emitting element manufacturing method according to the present invention comprises the steps of forming at least a light-emitting layer on a semiconductor substrate, and processing an edge of the semiconductor substrate to round the edge.
A semiconductor light-emitting element manufacturing method according to the present invention comprises the steps of forming a buffer layer on a first transparent semiconductor substrate, forming a Bragg reflective layer on the buffer layer, sequentially forming a light-emitting layer, a cladding layer, and a bonding layer on the Bragg reflective layer, forming a photonics crystal layer on a second semiconductor substrate, bonding the cladding layer and the photonics crystal layer via the bonding layer, and removing the second semiconductor substrate.
A region having a gradient index inside the photonics crystal layer or semiconductor layer is formed on one surface of the light-emitting layer. Thus, light emitted by the light-emitting layer can be efficiently extracted outside the element to increase the extraction efficiency and luminance.